The present invention relates to a non-contact on-line inspection method and apparatus for the grid pitch size, solder ball height, diameter, missing ball and coplanarity of a ball grid array (BGA) package, which are being widely employed in the microelectronics industry. The fast flash phase shifting method and the novel structural design of the present invention provide for the fast calculation of various parameters at a very fast speed and with high precision and accuracy.
In the microelectronics industry, chip makers improve chip performance by using ever smaller line widths to pack millions of transistors on a chip. Today more than three million transistors are packed in a chip, and the number is expected to increase to one hundred million in the near future. In turn, the packaging technology must provide more input/output connections with a finer pitch to accommodate the new chips. Conventional packaging techniques cannot meet this requirement. New advanced packaging techniques are being proposed and studied. The ball grid array (BGA) is one of the latest packaging techniques, and it has been studied widely in recent years. A ball grid array (BGA) is an array of solderable balls placed on a chip carrier. The balls contact a printed circuit board in an array configuration where, after reheat, they connect the chip to the printed circuit board. BGA""s are typically used in 40, 50, and 60 mil spacings with regular and staggered array patterns. So far, this kind of package has been utilized by industry, and it has been established as part of an industrial standard, which also includes such things as ball count, ball dimension and height, coplanarity, lead pitch, body size, etc. These specifications are very important in product quality control. They greatly effect reliability. For example, defective balls adversely affect the overall reliability of the bonds created by the soldered ball process. If a ball has an insufficient height, then no electrical connection is made between the ball and the corresponding pad of the circuit board. On the other hand, if a ball has excessive height an undesired connection may occur.
The separation between the ball tips and the flat surface are referred to as coplanarity. Generally, if a chip is placed on a flat surface, such as a printed circuit board, only three balls will make contact with the board (three points defined a plane). Other balls may contact or nearly contact the PCB surface depending on chip tolerances, on the size of the balls, on the flatness of the chip, etc. The closer to the surface all the balls are the more reliable will be the interconnection of all the leads once the solder has been reflowed. The magnitude of the coplanarity parameter is a predictor of how well chip leads can be accurately and reliably soldered to their corresponding pads on a printed circuit board and as a diagnostic measure for determining when the elements of the package fabrication process are going out of control. All these issues present important problems for non-contact on-line inspection.
The ball position, ball diameter and package dimension can be obtained by means of an image processing technique. The most difficult task is to measure the ball height. Some prior art methods of non-contact and contact measurement have been developed, but there are some shortcomings in these prior art techniques. Therefore, it is an objective of the present invention to provide a sufficiently high, rapid and accurate method and apparatus for measuring BGA characteristics.
U.S. Pat. No. 5,652,658 describes an optical, non-contact system for measuring the height of a BGA solders ball. The system uses a three dimensional scanner, which works on a triangulation principle, to gather data which is analysed to determine height. The grid array to be scanned is placed upon a fixture above a motion control table. The motion control system is controlled by a personal computer, and it has a high resolution. An automatic pick and place device is used to load the grid array packages on the fixture. The fixture consists of a plate with a cavity having the size of the grid array package being inspected. In addition to the height and position measurement, the measurements of grid pitch, ball diameter, ball position, and coplanarity can be performed. The problems with this arrangement include low efficiency due to the adoption of a point scanning method, and a high cost due to the requirement for a high resolution moving stage and a high-speed data acquisition system.
U.S. Pat. No. 5,815,275 describes a method and system, which utilizes a triangulation based laser line scanning principle for BGA measurement. A scanning beam is incident at a normal angle to the X, Y inspection plane with a scan line oriented at a 45 degree angle. The motion of the imaging head along the axis is used to acquire line scan images. The problem with this technique is a low efficiency due to the requirement for a high resolution moving stage.
U.S. Pat. No. 5,574,688 describes a contact method for measuring ball grid arrays. In this method, an array of touch sensors is brought in contact with a ball grid array. A linear actuator moves the array of touch sensors. The position of the linear actuator is known very precisely. As the touch sensors encounter the ball grid array, they provide a signal to a computer system indicating that an element on the ball grid array has been encountered. The computer notes the position of the linear actuator and the particular sensor in the touch sensor array that is providing the signal. The problem with this arrangement is a low efficiency due to the requirement for a mechanical movement structure, and a high cost due to the requirement for a high resolution moving stage. In addition, the rigid structure of sensor array cannot accommodate the different kinds of BGA patterns.
The field of three-dimensional methods includes useful techniques for on-line inspection such as projected grating and the use of a shadow moirxc3xa9. The shadow moirxc3xa9 techniques are simple and cost effective, but apart from this, the measurement is performed very close to the actual location of co-planarity. Classical shadow moirxc3xa9, therefore, cannot be applied directly due to its low accuracy.
In the present invention, a novel flash phase shifting method combined with a computer aided shadow moirxc3xa9 technique is used to obtain quantitative height data for a BGA. The novel phase shifting technique and algorithm enable the present invention to do on-line inspection of a BGA and overcome the above-described problems of the prior art.
More specifically, in the present invention, a plurality of light sources with the same specifications is employed. Preferably, there are six light sources arranged in a line. The illumination from the light sources is incident on the surface of an integrated circuit or chip at known angles. A test chip is placed on the surface of a uniformly space grating. A recording camera is preferably positioned on the other side of the grating to view the chip through the grating. The light sources are positioned so that the line between them and the center of the test object surface and the optic axis of the camera form a plane. The lines of the grating are normal to this plane. The method and apparatus of the present invention relies on the shadow of the grating lines to interfere with the grating lines themselves to produce fringes known as the moirxc3xa9 effect. The light sources flash and an imaging device captures an image of the illuminated object. The image data is processed according to a novel algorithm. A relatively fast and highly accuracy on-line inspection of a BGA can be achieved. This invention is not only suitable for BGA inspection but, it is also applicable to fast non-contact three-dimensional profilometry as well as flatness measurement.